The link between tau, a microtubule binding protein whose function is regulated by phosphorylation, and the pathogenesis of Parkinson's disease (PD) remains unclear. In view of this, it is interesting that mutations in the gene encoding Leucine-rich repeat kinase 2 (LRRK2), which are a major risk factor for PD, have been associated with pathologies that include tau tangles and aberrant tau phosphorylation. The most common mutation, G2019S, is within the kinase domain of LRRK2 and shows increased kinase activity in vitro, which is proposed to mediate its cellular toxicity. The normal function of LRRK2 and how mutations affect this function to affect disease pathogenesis is unknown. Tau has recently been shown to be phosphorylated by LRRK2, but it is still not known if LRRK2 and tau interact in vivo and how this interaction is altered by mutations in LRRK2 to affect aggregation and cause disease. The overall objectives of this application are to determine if LRRK2 modulates tau pathology and to identify the mechanism(s) by which this occurs. This proposal will use a combination of in vitro and in vivo approaches to identify mechanisms by which tau phosphorylation and aggregation are altered by wild-type (WT) and mutant LRRK2 and determine how this affects neurotoxicity. Experiments under Aim 1 will determine how tau phosphorylation and aggregation are altered by WT and mutant LRRK2 in vitro and in cell culture systems. Preliminary data has confirmed that tau is a good substrate for LRRK2 kinase activity and has identified multiple sites that are phosphorylated by tau in vitro. The effects of LRRK2 phosphorylation at these sites on the development of tau aggregates will be examined using tau inclusion formation in vitro and in cultured cells. The biological relevance of data generated in Aim 1 will be tested by studies under Aim 2 of this proposal that will determine how WT and mutant LRRK2 modulate tauopathy in novel mouse models. WT and G2019S LRRK2 mice will be crossed with the rTg4510 mouse model of tauopathy, which has a well-characterized disease course of tauopathy. Tau phosphorylation, aggregation and neurotoxicity will be measured in tissue from these mice. Preliminary data show that WT LRRK2/rTg4510 have increased tau phosphorylation at specific sites and increased tau aggregation, supporting the use of these models in this proposal. Results from these studies are essential to understanding PD etiology and directing future research to identify mechanisms of LRRK2-mediated tau pathogenesis.